A composite material system is a material system that includes materials which consist of different elements, but have the same band gap. For example, Al.sub.x Ga.sub.1-x As has the direct band gap energy of 1.42 eV to 1.98 eV for 0 &lt;x &lt;0.45. Ga.sub.x In.sub.1-x As.sub.y P.sub.1-y lattice matched to GaAs has the direct band gap energy of 1.42 eV to 1.89 eV and AlGaInP lattice matched to GaAs has the direct band gap energy higher than 1.89 eV. Therefore, AlGaAs/GaInAsP or AlGaAs/AlGaInP forms a composite material system.
Prior art laser diodes use only one single material system where the deposited layers which define the laser are of the same material system except that the layers include the various compositions of the elements which adjusts the energy gap profile of the layers as is well understood. Thus the familiar material systems include, for example, GaAs/AlGaAs, GaInAsP/GaInP and GaInAsP/AlGaInP.
In diodes where a single material system is used, the refractive index is a single function of the band gap of the material. That is to say, the lower the refractive index, the higher the band gap.
For the composite material system, the refractive index is not only a function of the band gap, but also is a function of material system. FIG. 3 shows the refractive index of AlGaAs and AlGaInP, which belong to different material systems. For the same bandgap energy, the difference of refractive index between AlGaAs and AlGaInP can be as large as 0.08. This difference in refractive index is enough to achieve some laser structures with preferred characteristics.